Case hardening steel processes

I’m looking for some help with case hardened sheetmetal parts. We have some failed sheet metal parts – 1mm thick C-1018 steel – that have cracked in the corners, and I suspect a change in the process from a matte black Nitrotec (nitrocarburize) to some other hardening process may be the cause. The new process has a glossy almost painted like finish.

When I try to measure the surface hardness, I get readings all over the map from 15-55 Ra to 60-75 R15N. I’m familiar with the nitrocarburizing and case hardening, but other processes that impart this glossy finish is where I need some help, and have a couple questions.



Which processes impart that glossy finish, and what are the hardness characteristics (Rockwell, depth, etc.) of that finish? How best to measure the hardness – Rockwell 15N, Ra, or other? Does the finish affect the hardness reading at all?

Thanks Much...
Newbie to this forum. : )

The case hardening could be an issue however small radii corners or often near sharp corner bends are usually the stress riser. Can you post a picture with radius dimensions for review?

Here's a couple of pix - the first showing what should be adequate bends for 1018, and the second showing the difference in glossy vs. matte finish. The matte parts didn't have problems, and the glossy parts crack on both burr side and opposite side.
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[ATTACH]2271[/ATTACH]

Looks like small radius syndrome to me..

Moreover, the cutout diameters are really close to the bend. The forming operation creates a high stress region, tension at outer R and compression at inner R. Anything you can do to get the assembly loading off the radius area the better. Simply, you most likely need less fatigue/loading stress at the crack area.

Have you run a stress analysis on that?

On the "crack" image look at the distorsions on the face with the thru holes...

Confront the vendor and ask about the change in finish.

Thanks to everyone for the feedback, but please keep in mind:

1) The matte finish parts which are spec'd at nitrocarburize 51-64 R15N have worked successfully. Yes, there are holes near the bends, and the bends reverse direction from the burr side, so these are indeed stress risers, but the parts did work for their intended life.
2) Something in the process changed to give the glossy finish, and the hardness readings vary widely.
3) Yes, we've asked our offshore supplier, but communication has been slow due to Spring Holiday, but we also heard the Chinese MEE has been cracking down on industry, and they've had to change things - to what, we don't know.

That's why I'm here to explore all avenues, and see if anyone has experience with all these surface hardening processes. Aside from the nitrocarburize that I'm familiar with, there's also carbonitriding, salt bath nitriding, gas nitriding, and a few others. What I'm hoping for is that someone can tell me which coating it likely is based on the wide range of hardness, and the glossy finish.

Thanks in advance...

...PS - Does someone know of a good resource that explains the difference in all these processes?

Thanks again.

Curious though ... why case hardening as opposed to heat treating?

This is 1mm (.039 in.) thick stock, so the case depth is .001-.0015 which gives a very hard wear surface while retaining a softer core. Through hardening is not possible in the 1018 material, and a higher carbon steel would twist and curl like a pretzel.
Some of these other surface hardening processes can give deeper cases,so that likely explains the wide ranges I've been measuring.

I guess the nuts and bolts of my question is how best to measure and sort if I'm getting such a wide range through two Rockwell scales? Anyone have experience with ultrasonic measurements??

Have you verified the base material used in these parts yet? I assume there is not material trace-ability required.